Colour Theory
by QuoteMyFoot
Summary: Jirou has synaesthesia and has heard colours in every voice since before she can remember - but it's Momo's colours that she falls in love with. A minor AU where Jirou has sound-colour synaesthesia.


Colours had fascinated Jirou for as long as she could remember. She would beg her parents to get her those paint catalogues they handed out at DIY stores, pouring over the gradients of colour and memorising their individual names. Not just simplistic; red, yellow, pink. Duck egg blue. Mint green. Coral.

Jirou hears colours in every song, every note, every voice and murmur and miscellaneous city noises. She knows the car backfiring in the street isn't Sojiro-san's ancient, terminal vehicle, because the noise is black and angry and forced instead of the sputtering but friendly grey of Sojiro-san's car. The city is full of bright neon lights, and she answers back to excited drunks who shout in those friendly colours, just another part of Tokyo.

It's when the voices turn leery and dark, muted greens and browns and nearly-black greys, that Jirou becomes wary, turning up the pop song in her headphones to drown them out. It's English and she doesn't even understand the lyrics, but the voice and notes and tempo make sunshine yellow in her ears, which is a colour that Jirou has secretly always loved.

She understands by middle school that this isn't usual, that most people don't _hear_ colours. Her parents, devotees of punk at heart, are confused but accepting when Jirou says she likes to keep a collection of pop songs in yellow and orange and pink for when she needs a pick-me-up; their voices turn maroon with an undertone of anger when she yells at Auntie to stop putting her parents down with backhanded compliments and the only explanation she has to offer is the words being a sickly, unnatural shade of green from Auntie's jealousy.

Insisting to her parents that real compliments are honey-coloured, warm and comforting amber, only makes them take her to a doctor.

It turns out there is a word for what Jirou is, but she doesn't like it. It's a clinical, steely grey that doesn't capture anything of the magic Jirou hears. She much prefers the song from an old English film, once she translates the lyrics.

Jirou doesn't have synaesthesia; she just hears the colours in the wind.

* * *

It would be going too far to call the UA entrance exam a breeze, but Jirou manages to clear it easily. The robots have their own distinct sound, the colours subtly different when they're weakened and nearly defeated, and she snipes a few points as well as listening out for the moss green hue of fear to rescue her would-be classmates.

She didn't _know _that would earn her points, but there was an undercurrent of deceptive sea blue in Present Mic's voice which made her think they weren't being told everything, and, well—didn't it make sense that acting heroic would be looked on favourably at a school for heroes?

So Jirou gets into UA, the school of her dreams. Her parents voices burst with sunshine yellow as they scream and dance around the house, and Jirou can't help but smile herself and listen, for days on end, to the happiest, brightest colours in her music library.

_I'm going to be a real hero!_

* * *

Jirou thought the heroics would only start when she was finished with high school, but then she finds herself in a fight for her life in their first training session. Not at all what she expected.

It feels easy. She hears the bright orange of her own voice echoed in Yaoyorozu and eventually Kaminari—a relief to see him returned to the colour she already associates with him, not dulled even when he shocks himself into stupidity. Jirou doesn't know whether to be glad he doesn't get a chance to leer at the half-naked Yaoyorozu or whether to be exasperated at his mindless confidence.

It all turns to horror when that mindless confidence gets projected back at them—a villain in hiding who takes Kaminari hostage. Jirou tries to sound bold, but she can't rid her voice of the taint of moss green and maybe that's why the villain sees her attempt at a counter-attack coming. Jirou's throat closes up from helplessness, from fear, from frustration, an ugly mix of pale pink and deep green and violent, shocking blue.

A miracle of timing saves them. Jirou hears a flood of sickly beige as Yaoyorozu sinks to her knees, nearly in tears. "We shouldn't have gotten away with that."

Jirou is equally relieved, and more than a little upset with herself for being unable to do nothing. "But we did. This time."

Yaoyorozu nods solemnly. Jirou wants to hug her, but she hesitates, arms held awkwardly at her sides.

It's Yaoyorozu who makes the move first—not then, but later, when they've seen that all their classmates are safe and the teachers have bluely fussed over them enough to take the edge off their nerves. Yaoyorozu wraps her arms around Jirou, squeezing all the breath out of her.

"We all made it," she says. "It's okay."

They've only been classmates for a scant few weeks, and normally, Jirou would never think of herself as someone who could friends with a girl as smart and stunning as Yaoyorozu. But a life or death situation has a way of bringing things into perspective.

Jirou hugs her back, and it's only then that the whole thing feels _finished, _that she feels safe enough to cry.

* * *

The USJ attack still lurks at the back of Jirou's head, a nightmare she feels like she has yet to wake from, even as school life goes back to normal—as normal as it ever is at UA. Jirou gets used to shoving the memories to the back of her head until Yaoyorozu approaches her after school one day.

"You said something a little odd during USJ," she begins without preamble. "You said you knew we had them on the run because they sounded green."

Jirou's heart sinks. She honestly doesn't remember saying such a thing, but the whole fight faded into a blur of movement and dull-coloured noise after the adrenaline faded. After the doctors told her parents she had synaesthesia, they were a lot more comfortable with Jirou describing noises as colour at home; in the heat of the moment, it's not that surprising that it burst out of her reflexively.

She expects to field half a dozen strange questions and have to explain for the millionth time that _no, _it's not a quirk, just another classmate who will forever think of her as 'the weird girl'—

But Yaoyorozu's eyes light up. She ducks her head in shyness when she asks, but is unable to conceal the warm yellow of delight in her voice: "Do you… have synaesthesia?"

Jirou blinks, taking a moment to register what she said. The word doesn't seem so bad, said in such a bright colour, and Jirou is able to say, "Yes," without reservation for once.

Yaoyorozu gasps, still coloured yellow—excited, not horrified or shocked. "That's so amazing! I've read about it! It comes from the greek words for..."

A litany of facts pours out of her, happy yellows tipping over excited red and the keen, burning curiosity that Jirou thinks of as 'Yaoyorozu's colour'—a brilliant, pure white. She's full of things that Jirou never knew about synaesthesia, something that she's always liked to think of as _her _own special thing; something that not just _anyone _had, like a quirk, but a thing which made her truly unique.

Now she hears about Franz Liszt who wanted his orchestra to play violet instead of rose; Richard Feynman who saw colours in numbers and letters, instead of hearing them; Marilyn Monroe who could see frequencies when she heard music…

But instead of making Jirou feel like just one of the crowd, as she expected, it makes her heart flutter. Maybe it's not unique, but she can't help but be warmed by how many musicians hear colours like she does. It makes her feel less of an idiot for being unwilling to give up on her hobby when the UA Heroics course is so notoriously difficult—for being unable to say goodbye to the rainbow of music. All the most beautiful colours and all the ugliest ones, just like the world those songs reflect. How could she let it go?

"Oh no," Yaoyorozu says, bright colours dimming into a burnt umber of embarrassment. "I went on for too long."

"No. It's good," Jirou replies bluntly, too embarrassed herself to say much more. She wants Yaororozu to know how pleased she is, though, so she adds, "I've never met anyone who knew what it was before. It was just… a weird thing I did."

"It's not weird at all; it's wonderful!" Yaoyorozu's voice is pale-pink-shy. "Do you mind… if I ask you about it a little?"

Yaoyorozu's voice is full of lovely colours. Jirou could listen to it all day. "I'd like that," she says.

Yaoyorozu's earnest interest in Jirou's synaesthesia keeps making her blush. She is _definitely _not used to receiving this many compliments from a pretty girl.

* * *

Of course, synaesthesia isn't the only thing that Yaoyorozu talks about. When she finds out that Jirou can play instruments, she engages her in a spirited discussion on music theory—which Jirou contributes to only clumsily. But she can't resist putting in her own contributions about the history of rock music when Yaoyorozu turns out to be classically trained and doesn't even know who _The Sex Pistols _are.

Somehow, this turns into Yaoyorozu coaxing Jirou into singing and playing guitar for her. Jirou can't refuse her excited smile, even if she's red-faced at taking Yaoyorozu to the apartment she and her parents live in—tiny compared to Yaomomo's enormous place—and heat radiates off her face in waves when Yaoyorozu can't take her eyes off her for the whole 'concert'.

"Jirou! Your voice is so beautiful!" she says, clapping, and Jirou could just about _die _right then. But die happy, at least.

"Th-thanks," she manages to reply.

"You said the songs all have colours?" Yaomomo asks. "What colours do these ones have?"

"Um… well..." Jirou has embarrassed herself enough already without chasing her away; she might as well just come out and say it. "I tried to pick songs that match the colours I get from you a lot—yellow and pale pink and white."

To her surprise, this time Yaoyorozu blushes. "I'm honoured." Her voice is tinged with a vibrant purple that makes Jirou's stomach somersault. She can't quite believe what she's hearing, but the colour of royalty certainly suits Yaoyorozu down to the ground. "Are they good colours?"

"Yeah," Jirou says. "Yeah. They're _your _colours."

She feels like she could float away when Yaomomo looks pleased.

* * *

Even as things settle back into a routine and her new friendship with Yaoyorozu goes from strength to strength—"I like her!" Dad had said after the first visit, making Jirou blush as Mom sighed; "At least wait until she's out the door, Kyo!"—Jirou tries to remember that feeling of helplessness from USJ and lets it carry her through the sports festival and into work experience: a reminder, a motivator. She feels herself relax, bit by bit, using her earjacks not only to locate the villains, but to detect their tone of voice—are they confident? Then they think they're in the clear the hero isn't expected. Are they anxious and scared? Then they're expecting the heroes to come, and the heroes must tread extra carefully and prioritise the hostages.

Remembering Yaoyorozu's encouragement, and the recurring vibrant purple in her voice which sends a thrill through Jirou every time she hears it, she even has the courage to explain her synaesthesia to the heroes, to show how she knows. They don't question it or call it weird, just nod thoughtfully and remark how useful it is for a hero with Jirou's quirk.

She practically glows. They called her a _hero._

Even after hearing of everything that went down in Hosu, the villain attack where Todoroki, Midoriya, and Iida were working, it's something that lifts her spirits as they go back to their ordinary lives at school.

"Did he really say that to you?" Mina says, as they share stories about work experience.

"I'm not surprised." Yaoyorozu nods decisively, even when Mina pokes her and protests that she 'didn't mean it like that!' "Of course Jirou would excel. Her quirk is perfect for that kind of situation!"

Jirou knows that Mina didn't mean it badly, but there's still something buoying about Yaoyorozu's defence. If someone so amazing talks about her being a hero like it's expected, like it would be wrong if they _hadn't _said that about her, fills her with joy— hearing the certainty and earnestness in Yaomomo's voice becoming the colour of honey, something she's always known to mean the warmest of compliments.

Yaomomo and Midoriya ask her for a detailed recounting of the hostage situation again and again, and Kaminari takes it upon himself to add embellishments—"excitement," in his own words—to big up her contribution.

"It was really a small thing," she tries to insist.

"No, no," Midoriya says, extremely unhelpful to her protests that she wasn't so important, but earning him a high five from Kaminari that goes unanswered because Midoriya is too busy muttering and scribbling in his notebook. Jirou now has a double-page spread to herself. "Information gathering is one of the most difficult parts of hostage negotiations. Even if it's possible to use monitoring equipment, the picture or audio is distorted and of course gauging the emotions of the villains is unreliable. But I've read up on sound-colour synaesthesia—"

"When?" Yaoyorozu asks.

"During lunch break," Midoriya answers, and Jirou isn't sure if Yaomomo is impressed or offended. She has to bite back a laugh at the adorable frown on her face. "Anyway I read that synaesthetes' perception of colour is usually affected by pitch which combined so Jirou's form of synaesthesia is quite rare and with her sensory related quirk means..."

Probably only Yaoyorozu can follow the muttered lecture that comes next, but the way she nods along, meeting her eyes and grinning like a cat who got the cream as Jirou's face turns a bright, unflattering red at the excessive praise.

Nonetheless, Jirou is pleased. She can't help it that she wants to spend more time with Yaomomo than ever. Even better, she doesn't seem to mind at all, not even when Jirou uses the 'Yaomomo' nickname out loud.

"It's nice," she says. "But you could call me just Momo if you wanted."

That same lovely purple curls through her voice and fills Jirou's chest with warmth, despite the butterflies in her stomach. She dares to hope that maybe the purple isn't a one-off, a passing feeling only. Maybe she really could have a chance with Momo?

"You can call me Kyouka too, then," she tells her.

* * *

She's afraid, and so is everyone else. She can hear their fear, confusion, panic, determination, outrage—the colours all swirl together in her head, churning in her gut and making her feel dizzy. Jirou coughs and the throat feels raw, each breath like sandpaper running across it, and she realises that the sickness isn't just the overwhelming colours, but a poison, a gas…

But Jirou can already feel herself slipping away.

—And she springs awake with a start, dazed and confused. What happened, where is she? White walls, white ceiling, a breath mask and…

Oh. It's a hospital. Jirou sinks back into the sheets, but it takes a long time for her heart to stop hammering in her ears like a drum, panic-yellow.

A nurse comes by soon enough. Jirou startles her by reaching out to grab her arm. "What happened?" she croaks. Her throat feels like it's being forced through a grater when she speaks, but she has to know. "Is everyone…?"

"Don't talk!" the nurse says, her chiding coming out with the soft pink of concern, the colour of the eyeshadow Jirou's grandmother always used to wear. "Your classmates are all fine now, as are your teachers. I'll… let them explain the rest to you."

Jirou wants to press for more, but the nurse rushes off to get her parents, and her throat _really _hurts. She has to content herself with wordlessly reassuring her parents worried faces and pretending that her father's singing voice doesn't keep cutting out as he chokes back tears.

"Music will make you feel better," he insists, when Jirou tries to wave him off. "This is one of your yellow ones, right? Right?"

The way Dad sings it then, it comes out in a mild meadow-green, _love, _with an undercurrent of icy-blue revealing how much he still worries. Only the guitar music sounds like the song, the same yellow notes as always.

Right then, it's a perfect mix of complementary colours.

* * *

Aizawa-sensei visits her briefly, but he's incredibly cagey about what happened at the training camp. All that he tells her is that Bakugou was kidnapped by some villains but that he was rescued.

She would accept that as the 'no big deal' Aizawa-sensei was trying to imply if she couldn't hear the undertone of burnt umber. For some reason, he's nervous.

It makes Jirou nervous too, especially when Kaminari insists on joining Mina to walk her home from the hospital.

"Yaomomo hasn't been to see you, has she?" is what Mina opens with, and Jirou's stomach just drops out from under her.

She gets the whole story from them in excruciating detail – they heard it from Kirishima and, in curt snippets, from Bakugou, so they know more or less everything.

Jirou wishes that Momo was there so she could scream at her, because her first reaction is fury and fear. _How dare you risk yourself without telling me? How dare you not even __**think **__of me? Of what it would be like for me to wake up and hear that you were dead because of some rescue expedition the pros were already handling?_

But the anger dies in her throat after a few minutes, because it's not really Yaoyorozu that she's furious with. If she'd been in that position—well, Bakugou is a jerk, but he's Class 1A's jerk and Jirou has seen him tutor Kirishima and the others enough to know that there's a lot of good in him too.

It's herself that she's mad at, for being helpless again, for just lying there uselessly whilst her friends and classmates needed her. When Momo needs her.

When will she ever be a hero when someone needs her?

What if Momo really had…?

Her throat closes up. It's stupid to cry when the danger has passed, when Yaomomo is safe and sound again, when Jirou wasn't even awake to worry about her.

"Oh man," Kaminari says, "you got it bad, huh?"

"Not _now,_ for god's sake," Mina hisses, kicking his ankle.

Jirou lets out a laugh that turns into a hiccup. She's grateful for them being there, for coming to see her safe at home, because she does have it bad, and she thought—the purple—

But Yaoyorozu didn't come to see her at all.

* * *

Jirou can't bring herself to text, worrying about what the response might be, and Yaoyorozu doesn't send her anything either. They don't see each other again until the time comes to move into the new UA dorms, when they awkwardly bump into each other carrying boxes upstairs.

"Jirou," Yaoyorozu says carefully, her voice pained with burnt umber. "You're… you look okay."

"I am okay," Jirou answers. She doesn't say, _Not that you would know, because you've been ignoring me, _but it's implied strongly enough for Yaoyorozu to wince.

"Well..." She doesn't meet Jirou's eyes, searching for something to say. "I'm only a few floors up if… if you need anything."

Jirou has no idea how to take that. Is it a genuine offer or one only made out of politeness? It's Yaomomo, so she would say genuine, but she's not using 'Kyouka' any more, so who knows.

They stand there for another few moments in silence.

Yaoyorozu opens her mouth, and then closes it again without saying anything. Jirou wishes she wouldn't; even if it was sort of nonsense, she might be able to read the colours and understand what she's thinking.

In the end, Yaoyorozu just shakes her head. "Speak to you later."

The colours in it are mint green for nerves and chocolate brown, regret. Jirou's heart lifts a little even as Yaoyorozu walks away, because at least she _feels_ sorry.

Now if only she would say it.

It's not like Jirou has acted perfectly either. Maybe she should've made an effort to approach her sooner to clear the air. But a stubborn part of her can't help but think that Yaoyorozu had more things she should've said, and that she ought to be the one to say something first.

* * *

They can't avoid each other entirely, and Yaoyorozu doesn't even seem to try. She still invites Jirou to study sessions with the other girls, will share popcorn on movie nights since they both like toffee flavour, and once even lends her a spare tie one morning when Jirou can't find hers (Kaminari hid it as a joke, but then forgot where he put it).

It still feels like an especially awkward dance. Jirou wonders if everyone else has noticed and is just being kind enough not to mention it.

Or maybe they're just distracted by the far more awkward thing, that Jirou keeps talking in colours. Before moving into the dorms, she didn't do it so much around her classmates, apart from Yaoyorozu who saw her at home. But she's so used to referring to colours as shorthand for emotions, or certain genres of music, that's it's hard to remember to talk normally in the more relaxed atmosphere of the Class 1-A dorms.

Many of them know about her synaesthesia by now, at least vaguely, but she still gets reactions like:

"Huh? _Yellow_ music?"

"Oh, if you're feeling blue, then we can do something nice! ...Wait, what does blue mean then?"

"Pink? Oho, you _looove _him—_ow, _Mina!"

Finally, Todoroki just bluntly asks: "Why do you mention colours so much if no one else knows what they mean?"

His expression is only genuine, if mild, curiosity, and she knows he doesn't mean it as badly as it sounds, but Jirou can't help but wince. She knows it is strange – even most synaesthetes (thanks, Midoriya, for that word) probably don't talk about it so much. It's just that her parents, once they realised how much the colours helped her with her music, were eager to know everything about how she heard the world in colour. Dad used to take her to his band's rehearsals sometimes and ask her if the music was the right colour. It's just something she learned to share.

But before Jirou can reply, Yaomomo says, a little tartly, "_I _know what she means."

"You do?" Todoroki blinks at her for a moment and then shrugs. Apparently that was enough for him, because he turns back to his soba without another word.

Kaminari, however, shows a sudden interest. "For real?! Well then, can you translate for the rest of us?"

"Dude, if you need the phrase 'seeing red' translated for you, that's your own problem," Sero says, and they descend into bickering before Yaoyorozu can answer.

Her face carries a thoughtful expression for a while, however, and she spends the rest of the evening with her nose in a notebook.

She doesn't ask Jirou anything, but she watches Yaoyorozu carefully for the rest of the evening.

* * *

Later, after everyone has gone back to their rooms, Jirou gathers her courage and her beloved DIY paint charts and heads upstairs.

If she waits for Yaoyorozu to say _sorry, _she could be waiting a long time. But it occurs to her that in small ways, like defending her from a perceived slight, or going out of her way to make sure Jirou still feels included in her circle of friends, Yaoyorozu has been trying to say _something _for a while now—it's just that Jirou hasn't been listening.

Despite this thought, she nearly retreats several times on the way up. Two flights of stairs have never felt quite so long. She takes a deep breath before she knocks on Yaoyorozu's door.

The door opens hesitantly at first. Jirou catches a glimpse of Momo's mouth parted in surprise before the door opens wide and she smiles instead.

Jirou can feel herself going red already. How can Yaomomo look so cute in worn, overlarge pyjamas? It's not fair. She ducks her head in an effort to cover her embarrassment and thrusts the colour charts forward.

"I thought it would be easier if you could refer to a specific colour," she mumbles.

For a moment she doesn't dare look up, but she feels the colour charts being taken gently out of her hands and lifts her eyes.

"That would be easier," Yaomomo agrees, "if you… don't mind staying over for a while?"

Jirou manages to shake her head and step over the threshold.

It's all worth it when Momo's face lights up with joy and her eyes sparkle. But then she dims, and when she speaks again, her voice is coloured with regret and fear. "I… I'm sorry that I didn't say anything before. Or that I didn't come see you." She ducks her head. "I was worried you would be angry with me for doing something so drastic."

"I was angrier that you didn't come to talk to me about it yourself," Jirou answers. Before she can think about it too hard, she reaches out for Momo's hand, her heart skipping a beat when Momo's fingers curl around hers. "But… I realised that wasn't worth pushing you away. So. All is forgiven."

"I won't make that mistake again," Momo promises. She squeeze Jirou's hand. "Let's make a reference chart."

That sort of sentence shouldn't sound purple, but it does.

They work together for hours, bent over the paper with their shoulders touching. Yaoyorozu makes the paint with her quirk, although sometimes it takes a few tries to get the colour pigment just right. Jirou writes notes until her hand cramps up. They stay up way too late, and when Jirou eventually heads downstairs, she knows that school tomorrow is going to be hellish.

It was exactly what she needed.

* * *

Unsurprisingly, they both sleep in late, and don't have time to actually put the colour charts up until after school. Jirou is actually pretty proud of them. She used her fanciest handwriting, which actually looks alright, and Yaoyorozu organised them into positive and negative feelings _and _grouped them according to colour theory.

_...Is it weird to find organisational skills kind of cute? It's weird, isn't it._

But Momo looks so proud of her work, and the tiny, satisfied smirk she wears is kind of hot. Jirou is confident no one would hold it against her.

Kaminari does a double-take when he sees Jirou and Yaomomo putting up the charts. "Wait, wait, I wasn't serious!" He groans. "Oh man, now I feel bad. How long did it take you guys to do this?"

Jirou catches Momo's eyes, dark circles and all, and they both smile in unison.

"Ugh, now I have to do something _nice_ for you," Kaminari grouses. He wanders away, yelling, "Oi, Sato, how much do I have to pay you to make a cake for Yaomomo and Jirou?!"

_Maybe I should make a copy for Mom and Dad at home, too, _Jirou muses, taking a step back to admire their handiwork. _Dad's always getting worry and nervousness mixed up… _

"Do you like it?" Momo asks anxiously.

Jirou nods. "I helped you make it, didn't I?"

"Ah. Sorry." She bites her lip. "I worried that you were just… going along with it. You didn't talk about your synaesthesia much when we started school."

"I guess things were different then." Jirou didn't trust her classmates very much then. After seeing how hard everyone is working to being a hero, she feels differently about them now. And if she's being honest… "You helped, too. I never knew all that stuff about synaesthesia before! It feels like something that's… cool to have now."

Momo blushes and looks pleased. "It's part of what makes you _you, _Kyouka. That makes it special to me, too."

Then it's Jirou's turn to blush. She can't help but remember that there _is _one important colour that's missing from the charts, and she knows just what to do about it.

* * *

She can't get Yaomomo to make the paint this time, of course, so she has to order some of those tiny tester pots online. She ends up ordering half a dozen and _none_ of them are quite the right shade. It takes Jirou three days of experimenting with mixing colours, as well as a three more orders of tester pots, but she finally manages to find something matching the vibrant purple she hears in Momo's voice. She's kind of glad that she doesn't really hear colours in her own voice, and _especially _glad that Momo doesn't, because it would be kind of embarrassing to think how long Jirou has been speaking in purple. Finally, she's ready. Jirou isn't eloquent like Momo is, so she thought it was better to keep things short and simple.

_Purple is 'romantic fondness'._

_Can I take you out for coffee next week?_

At least she can be fairly confident that Momo will say yes.

* * *

**A/N: **Jirou's experience of synaesthesia here was inspired by a university friend. He had grapheme-colour synaesthesia (when things like letters and numbers have an associated colour) and was studying physics. He described how he could tell if his calculations were off in an equation because the colours would look all gross and wrong - unsurprisingly, he was awesome at physics. (He described a similar thing happening with pieces of music and he was a talented musician too!)

I got to thinking how that kind of 'instinct' with your synaesthesia would translate to a Jirou with sound-colour synaesthesia, and this fic was born. I couldn't resist adding some Yaoyorozu/Jirou though. My first time writing for this pairing. They're so adorable! 3 Let me know what you thought! I welcome concrit as well!


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